(1) Multiple Input Multiple Output (MIMO) Technology
The term “MIMO” is short for Multiple Input Multiple Output. Beyond conventional schemes using a single Transmit (Tx) antenna and a single Reception (Rx) antenna, MIMO uses a plurality of Tx antennas and a plurality of Rx antennas to thereby increase the transmission and reception efficiency of data. With the use of multiple antennas at a transmitter or a receiver, MIMO seeks to increase capacity or improve performance in a wireless communication system. The term “MIMO” is interchangeable with “multi-antenna”.
The MIMO technology does not depend on a single antenna path to receive an entire message. Rather, it completes the message by combining data fragments received through a plurality of antennas. Because MIMO may increase data rate within a certain area or extend system coverage at a given data rate, it is considered as a promising future-generation mobile communication technology that may find its use in a wide range including mobile terminals, relays, etc. With the growth of data communication, MIMO is attracting attention as a future-generation technology that may overcome a limit on transmission capacity that is almost reached due to the increased data communication.
(2) MIMO System Model
FIG. 1 illustrates the configuration of a typical MIMO communication system. Referring to FIG. 1, a simultaneous increase in the number of Tx antennas of a transmitter to NT and in the number of Rx antennas of a receiver to NR increases a theoretical channel transmission capacity in proportion to the number of antennas, compared to use of a plurality of antennas at only one of the transmitter and the receiver. Therefore, transmission rate and frequency efficiency are remarkably increased. Given a maximum transmission rate Ro that may be achieved in case of a single antenna, the increase of channel transmission capacity may increase the transmission rate, in theory, to the product of Ro and Ri in case of multiple antennas. Ri is a transmission rate increase rate.
MathFigure 1Ri=min(NT,NR)  [Math.1]
For instance, a MIMO communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna system. Since the theoretical capacity increase of the MIMO system was proved in the middle 1990's, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
Concerning the research trend of MIMO, active studies are underway in many respects of MIMO, inclusive of studies of information theories related to calculation of multi-antenna communication capacity in diverse channel environments and multiple access environments, studies of measuring MIMO radio channels and MIMO modeling, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc.
To describe a communication scheme in a MIMO system in detail, the following mathematical model may be used. It is assumed that there are NT Tx antennas and NR Rx antennas as illustrated in FIG. 1. Regarding a transmission signal, up to NT pieces of information can be transmitted through the NT Tx antennas, as expressed as the following vector.
MathFigure 2s=└s1,s2, . . . , sNT┘T  [Math.2]
A different transmit power may be applied to each piece of transmission informations1,s2, . . . , sNT.
Let the transmit power levels of the transmission information be denoted byP1,P2, . . . , PNT,
respectively. Then the power-controlled transmission information
may be given as the following Math FIG. 3.
MathFigure 3=[,, . . . , ]T=[P1s1,P2s2, . . . , PNTsNT]T  [Math.3]
may be expressed as a diagonal matrixP
of transmit power.
MathFigure 4
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                                                        N                          T                                                ⁢                                                                                                                                                                                  ]                                =          Ps                                    [                  Math          .                                          ⁢          4                ]            
Meanwhile, actual NT transmitted signalsx1,x2, . . . , xNT 
may be configured by applying a weight matrixW 
to the power-controlled information vector.
The weight matrixW 
functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel statuses, etc. These transmitted signalsx1,x2, . . . , xNT 
are represented as a vectorX, 
which may be determined as
MathFigure 5
                    x        =                              [                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                    [                                                                                                    w                        11                                                                                                            w                        12                                                                                    …                                                                                      w                                                  1                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    …                                                                                      w                                                  2                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      w                                                  iN                          t                                                                                                                                                ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            …                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                j                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                  s                  ^                                            =              WPs                                                          [                  Math          .                                          ⁢          5                ]            Wij 
denotes a weight for a jth piece of information transmitted through an ith Tx antenna.W 
is also referred to as a precoding matrix.
Given NR Rx antennas, signals received at the Rx antennas,y1,y2, . . . , yNR 
may be represented as the following vector.
MathFigure 6y=[y1,y2, . . . , yNR]T  [Math.6]
When channels are modeled in the MIMO communication system, they may be distinguished according to the indexes of Tx and Rx antennas and the channel between a j th Tx antenna and an ith Rx antenna may be represented as hij. It is to be noted herein that the index of the Rx antenna precedes that of the Tx antenna in hij.
The channels may be represented as vectors and a matrix by grouping them. The vector representation of channels may be carried out in the following manner.
FIG. 2 illustrates channels from NT Tx antennas to an ith Rx antenna.
Referring to FIG. 2, the channels from the NT Tx antennas to the ith Rx antenna may be expressed as the following Math FIG. 7.
MathFigure 7HiT=└hi1,hi2, . . . hiNT┘  [Math.7]
Also, all channels from NT Tx antennas to NR Rx antennas may be expressed as the following matrix.
MathFigure 8
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                                                  T                          ⁢                                                                                                                                                                                                                          ]                                              [                  Math          .                                          ⁢          8                ]            
Actual channels experience the above channel matrix H and then are added with Additive White Gaussian Noise (AWGN). The AWGNn1,n2, . . . , nNR added to the NR Rx antennas is given as the following vector.
MathFigure 9n=[n1,n2, . . . , nNR]T  [Math.9]
From the above modeled equations, the received signal is given as
MathFigure 10
                                                        y              -                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                                              N                            R                                                    ⁢                                                                                                                                                                                                    ]                            -                                                [                                                                                                              h                          11                                                                                                                      h                          12                                                                                            …                                                                                              h                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    h                          21                                                                                                                      h                          22                                                                                            …                                                                                              h                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                h                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              h                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                                                    N                              R                                                        ⁢                            1                                                                                                                                                h                                                                                    N                              R                                                        ⁢                            2                                                                                                                      …                                                                                              h                                                                                    N                              R                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                              x                          1                                                                                                                                                              x                          2                                                                                                                                    ⋮                                                                                                                                      x                          j                                                                                                                                    ⋮                                                                                                                                      x                                                      N                            T                                                                                                                                ]                                            +                                             ⁡                      [                                                                                n                    1                                                                                                                    n                    2                                                                                                ⋮                                                                                                  n                    i                                                                                                ⋮                                                                                                  n                                          N                      R                                                                                            ]                          -        Hx        +        n                            [                  Math          .                                          ⁢          10                ]            
(3) Channel Quality Indicator (CQI)
For efficient communication, it is necessary to feed back channel information. In general, information about a downlink channel is transmitted on an uplink and information about an uplink channel is transmitted on a downlink. This channel information is called a CQI. The CQI may be generated in various manners.
For example, the CQI generation methods may include transmitting quantized channel state information a CQI, transmitting the Signal to Interference and Noise Ratio (SINR) of a channel as a CQI, and transmitting information indicating how a channel is actually used, such as a Modulation and Coding Scheme (MCS) level, as a CQI.
Many of the CQI generation methods generate a CQI based on an MCS. For example, a CQI is generated based on an MCS in 3rd Generation Partnership Project (3GPP) High Speed Downlink Packet Access (HSDPA). An MCS specifies a modulation scheme, a coding scheme, and a coding rate. Therefore, a CQI is changed if a modulation scheme and a coding scheme are changed and thus at least one CQI is required per codeword.
In a MIMO system, the number of required CQIs is changed. As the MIMO system generates multiple channels through multiple antennas, a plurality of codewords is available. Accordingly, a plurality of CQIs should be reported, thus increasing the amount of control information proportionally.
FIG. 3 illustrates the concept of CQI generation and CQI reporting. A User Equipment (UE) measures a downlink channel quality and reports a CQI value selected based on the downlink channel quality measurement to a Base Station (BS) on an uplink control channel. The BS performs downlink scheduling based on CQI reports received from UEs. The downlink scheduling involves selecting a UE and allocating resources to the selected UE. A CQI value may be any of SINR, Carrier to Interference and Noise Ratio (CINR), Bit Error Rate (BER), and Frame Error rate (FER). Alternatively, the CQI value may be a transmittable value as which one of the SINR, CINR, BER and FER is expressed. In the MIMO system, Rank Indicator (RI), Precoding Matrix Index (PMI), etc. may be added as channel state information.
(4) Characteristics of CQI in Frequency Band
In mobile communication systems, an MCS and transmit power are controlled for a given channel by link adaptation in order to fully utilize the capacity of the channel. To allow the BS to perform the link adaptation, the UE should feed back a CQI to the BS.
If a system frequency band has a bandwidth wider than a coherence bandwidth, a channel is changed greatly within one bandwidth. Particularly, a multi-carrier system such as an Orthogonal Frequency Division Multiplexing (OFDM) system uses a plurality of subcarriers in a given bandwidth and a modulated symbol is transmitted on every subcarrier. While it is optimal to transmit a channel per subcarrier, the amount of feedback channel information rapidly increases. In this context, many schemes have been proposed to reduce the control overhead.
(5) Concept of Multi-Carrier
The concept of multi-carrier and Component Carrier (CC) will be described below. FIG. 4 illustrates a frequency band in a multi-carrier system. Referring to FIG. 4, the multi-carrier represents a total frequency band available to a BS, equivalent to a whole band in its meaning. For example, the multi-carrier may be 100 MHz.
A CC is an element of the multi-carrier. That is, a plurality of CCs form the multi-carrier by carrier aggregation. The CC includes a plurality of lower bands. If a multi-carrier is called a whole band, a CC may be referred to as a subband and a lower band may be referred to as a partial band. Carrier aggregation is intended to extend a bandwidth by aggregating a plurality of carriers in order to increase data rate. For example, a legacy system uses a carrier of 20 MHz and the bandwidth of the carrier may be extended up to 100 MHz by aggregating five 20-MHz carriers. The term “carrier aggregation” covers aggregating carriers in different frequency bands.
How to report the above-described CQI in the multi-carrier system becomes an issue to be solved.